PURPOSE: To obtain a lightweight and small-sized piston engine at a low cost by interlocking a piston and a crankshaft through gears and reciprocating the shaft by means of intermittent rotations of a pulley and a force receiving metal.

CONSTITUTION: When a piston 2 is moved, a crankshaft 14 is rotated through gears 6W7, 9W10, a connecting rod 12, a crank 13. The shaft 15 of a sliding disk 4 is interlocked with a pulley, and shafts 18, 20 are reciprocated. When a spring 19 shrinked, the pulley 16 makes a half turn. Then, the piston 2 enters into a base plate 3, the sliding disk 4 makes a half turn, and a cylinder in an explosion stroke takes the place of a cylinder in a suction stroke. Accordingly, a suction valve and an exhaust valve are not required, and a lightweight and small-sized piston engine can be obtained.